Means for increasing the acoustic power output of underwater transducers

ABSTRACT

An underwater transducer is force limited and mounted in a baffle structure which presents less than a 100 percent rho-c loading. By so reducing the radiation resistance of the acoustic impedance of the transmission medium, the acoustic output from the transducer can be increased.

United States Patent Massa [451 Oct. 17,1972

i541 MEANS FOR INCREASING THE ACOUSTIC POWER OUTPUT OF UNDERWATERTRANSDUCERS [72] inventor:

[73] Assignee: Massa Division, Dynamics Corporation of America, Hingham,Mass.

[22] Filed: Aug. 5, 1970 [21] Appl. No.: 61,199

Frank Massa, Cohasset, Mass.

[52] US. Cl. ..340/8, BIO/9.1, 340/9 [51] Int. Cl. "601v 1/00 [58] Fieldof Search ..340/8i4; 3 10/91 [5 6] References Cited UNITED STATESPATENTS Massa, Jr. ..340/1 2 R 2,906,991 9/1959 Camp ..340/9 X 3,492,6341/1970 Massa ..340/9 3,019,661 2/1962 Welkowtiz ..340/8 R 2,923,7822/1960 Armstrong et al. ..340/8 R Primary Examiner-Benjamin A. BorcheltAssistant Examiner-W. J. Tudor Attorney-Louis Bernat [57] ABSTRACT Anunderwater transducer is force limited and mounted in a baffle structurewhich presents less than a 100 percent rho-c loading. By so reducing theradiation resistance of the acoustic impedance of the transmissionmedium, the acoustic output from the transducer can be increased.

12 Claims, 2 Drawing Figures MEANS FOR INCREASING THE ACOUSTIC POWEROUTPUT OF UNDERWATER TRANSDUCERS This invention relates to means forimproving and increasing the acoustic power output from underwatertransducers, and more particularly to an array of spherical transducers.

The following description uses the term rhoc which is well known inacoustic engineering. This term refers to the characteristic acousticimpedance of the medium (such as ocean water, for example) into whichthe transducer is driving. The description also refers to a sphericalshaped transducer which behaves as a dipole source of sound. This isdifferent from a pulsating sphere which acts as a point source.

Generally, it has been assumed that the optimum perfonnance for anunderwater transducer is realized when a full 100 percent rhoc loadingis achieved for the operating environment of the transducer. At thelower audio frequencies, this full loading condition is often achievedby assembling a large number of transducer elements into an arrayconfiguration. The array is usually configured so that the radiationresistance acting upon the transducers approaches the desired 100percent rhoc loading.

Another general assumption has been that a transducer increases itsability to generate acoustic power responsive to an increase of theradiation resistance. For the usual amplitude limited transducer, thepower output does increase as the acoustic loading on the transducer isincreased, and this is in accordance with the generally accepted belief.However, the general assumption is not necessarily true for all othertypes of transducer. For example, l have found that it is possible toreduce the radiation resistance below the 100 percent rhoc loadingfactor and, under certain circumstances, to actually increase theacoustic output from the transducer. The circumstances under which thismay be achieved includes the use of a transducer in which theelectromechanical drive system is force limited. This is in contrastwith a more conventional type of underwater transducer in which thevibrating system is amplitude limited.

Accordingly, an'object of this invention is to increase the acousticpower output from an underwater transducer. Here, an object is tocombine a force limited transducer with a baffle structure whichpresents less than a 100 percent rhoc loading on the radiating surfaceof the transducer.

Another object of this invention is to increase acoustic power output ofa transducer array. Here, an object is to mount a number of inertialtype electromagnetic transducers in an array configuration which resultsin less than 100 percent rhoc loading on the transducers. Still anotherobject of this invention is to increase the acoustic power radiationfrom an inertial type electromagnetic transducer by coupling thetransducer to an underwater horn which presents less than a 100%acoustic resistance loading to the vibrating surface of that transducer.

Yet another object of this invention is to increase the acoustic powergenerating capability of an array of inertially driven, electromagnetic,dipole transducers. In this connection, an object is to assemble a groupof such transducers into an array configuration in which open spaces areprovided in the array between adjacent transducers. Here, an object isto use the open spaces to reduce the resulting radiation resistance uponthe radiating surfaces of the transducers.

The structure for accomplishing these and other objects is set forthwith particularity in the appended claims.

However, for a better understanding of the invention itself, togetherwith further features, objects, and advantages thereof, reference ismade to the accompanying description and drawings, in which:

FIG. 1 is a schematic cross sectional view of an underwater horn coupledto a force limited transducer, the horn being designed to present lessthan a lOO percent rhoc loading to the transducer; and

FIG. 2 is a schematic view of an array of force limited inertial typetransducers, arranged in a configuration that reduces the eflectiveacoustic loading on the array, thereby increasing the acoustic poweroutput of the transducers.

In FIG. 1, a spherical transducer 11 is shown as being mounted into asmall opening at the end of an underwater horn 12. The underwater hornmay be constructed of concrete loaded with scrap steel for incre asingthe density of the horn structure. Details of a suitable underwater horndesign are given in U.S. Pat. No. 3,360,771, issued Dec. 26, 1967, andassigned to the assignee of this invention. The transducer 1 l is aninertial, electromagnetic, force limited, spherical transducer which maybe similar to the structure described in U.S. Pat. No. 3,319,220, issuedMay 9, 1967, for example, also assigned to the assignee of thisinvention.

The transducer is mounted in the small or rear end of the horn by meansof the brackets 13 and the bolts 14. The brackets 13 are preferablymolded into the peripheral edge of a rubber belt 15 which equatoriallysurrounds the transducer sphere 11. This belt serves to resiliently holdthe transducer in a fixed position, while enabling the vibrations of thespherical transducer to reach surrounding medium, when electrical poweris supplied thereto via a suitable cable (not shown).

The acoustic resistance at the throat D of an exponentially shaped homis proportional to l/A,,, where A,, is the area of the horn at thediameter D, shown in FIG. 1. If loaded with a percent rhoc loadingfactor, theacoustic resistance on the radiating surface of the vibratingsphere is proportional to llA where A, is the projected area of thesphere. Thus, if A is made greater than A,, the acoustic loading on thetransducer is less than 100 percent rhoc. This is assumed to be the caseillustrated in FIG. 1.

in greater detail, the diameter D of the horn in FIG. 1 is small ascompared to the wave length of the sound being radiated. Moreparticularly, an exponential horn has a throat diameter which must besmall compared to the wave length if the structure is to behave as ahorn. Here, the concem is to make the throat diameter D larger than thediameter of the transducer attached thereto. in this way, I achieve areduced acoustic load on the vibrating sphere and thereby increase theacoustic power radiated, provided the transducer is a force limitedvibrating structure.

Therefore, the diameter of the spherical transducer element is smallcompared to the wave length of sound being generated. If this were notso, there would be no point in having a horn. The acoustic radiationresistance on the surface of a vibrating structure approaches 100percent rhoc, for the medium, when the size of the vibrating structureis large in comparison to the wave length of the radiated sound.

If the throat diameter of the exponential horn were made equal to thediameter of the vibrating sphere, the acoustic impedance presented tothe transducer would be equivalent to 100 percent rhoc acoustic loading.on the other hand, if the diameter D were made less than the diameter ofthe transducer, the acoustic radiation impedance presented to thetransducer would be greater than 100 percent rhoc loading. This lattercase is typical of conventional types of horn design, and it isparticularly suitable if a horn is driven by an amplitude limitedtransducer. In other words, if the transducer were generating a constantamplitude, the acoustic power would increase if the throat diameter D ofthe horn were made less than the diameter of the transducer. This isexactly the opposite of what is here achieved by the use of a forcelimited transducer, as described herein.

The acoustic power is increased for a force limited transducer coupledto a horn. This increase in power occurs since the acoustic power outputof a force limited transducer is proportional to F IR where F is theelectromagnetic force generated in the electromagnetic drive system ofthe transducer, and where R,, is the acoustic radiation resistance load.The acoustic radiation resistance load R is made lower as the area ofthe horn at the diameter D (FIG. 1) is made larger. The result is anincreased level of radiated acoustic power. The electromagnetic, forcelimited transducer delivers higher power with decreasing radiationresistance loading since the amplitude of vibration of the transducerincreases to satisfy the fundamental equations of motion. The practicallimit upon this increase in output is imposed by the design of thevibrating system in the transducer and by the maximum permissibledisplacement of the structure.

The same conclusions cannot be applied to an amplitude limitedtransducer such as a piezoelectric transducer, for example, because, ata constant amplitude of the transducer, the acoustic power outputdecreases as R, decreases.

FIG. 2 illustrates another arrangement comprising a plurality oftransducers 11A, each of which is the same as transducer 11 in FIG. 1.The transducers are here used to give a reduced acoustic loading for thetransducers. The individual transducers are mounted with the sameelastic belt suspension system, as illustrated at 15 in FIG. 1. Eachtransducer 11A is mounted within a circular rigid sleeve member 16, aplurality of which are attached within a frame structure 17 by means ofweldin g as at 18.

The openings 19 between the adjoining sleeve members 16 extend throughthe array assembly. These openings may be either left open or partiallyblocked by filling the interstices with rigid structural barriers (notshown). If the openings 19 remain in the array, they reduce theeffective radiation resistance loading on the transducers. Thus, anincreased acoustic power is achieved from the electromagnetictransducers, for

the reasons described above in connection with FIG. 1. It should beunderstood that modifications and variations may be effected withoutdeparting from the spirit and scope of the novel concepts of thisinvention. Therefore, the appended claims are to be construed to coverall equivalent structures.

I claim:

1. A structure for radiating sound underwater, said structure comprisingat least one force limited transducer, means for increasing the acousticpower radiated from said transducer, said means including a bafflestructure associated with said transducer, said baffle structure havinga throat diameter relative to the diameter of the transducer whichpresents less than percent rhoc resistance loading to the transducer.

2. The invention in claim 1 characterized in that said transducer isdriven by a force limited system, of the inertial type.

3. The invention in claim 1 and means for electromagnetically drivingsaid transducer.

4. The invention in claim 1 characterized in that said transducer isdriven electromagnetically, and further characterized in that saidbaffle structure is a tapered horn.

5. The invention in claim 4 wherein said tapered horn is made of cementloaded with metallic parts.

6. The invention in claim 4 further characterized in that said taperedhorn has said throat area which is larger than the working area of saidtransducer.

7. The invention in claim 1 wherein said baffle structure includes aframe, a plurality of tubular sleeves mounted within said frame, and aplurality of transducers mounted within individually associated ones ofsaid tubular sleeves.

8. The invention in claim 7 further characterized in that saidtransducers are electromagnetic devices.

9. The invention in claim 8 further characterized in that saidtransducers are of the inertial type.

10. The invention in claim 7 further characterized in that there areunfilled openings between said tubular sleeves whereby the radiationresistance on the transducers is effectively reduced.

11. The invention in claim 10 further characterized in that saidtransducers are spherical electromagnetic devices, each having amounting structure comprising an equatorially positioned resilient belt,said belt being compressed between the wall of said sphere and saidtubular sleeve.

12. The invention in claim 11 still further characterized in that saidtransducers are of the inertial type.

1. A structure for radiating sound underwater, said structure comprisingat least one force limited transducer, means for increasing the acousticpower radiated from said transducer, said means including a bafflestructure associated with said transducer, said baffle structure havinga throat diameter relative to the diameter of the transducer whichpresents less than 100 percent rhoc resistance loading to thetransducer.
 2. The invention in claim 1 characterized in that saidtransducer is driven by a force limited system, of the inertial type. 3.The invention in claim 1 and means for electromagnetically driving saidtransducer.
 4. The invention in claim 1 characterized in that saidtransducer is driven electromAgnetically, and further characterized inthat said baffle structure is a tapered horn.
 5. The invention in claim4 wherein said tapered horn is made of cement loaded with metallicparts.
 6. The invention in claim 4 further characterized in that saidtapered horn has said throat area which is larger than the working areaof said transducer.
 7. The invention in claim 1 wherein said bafflestructure includes a frame, a plurality of tubular sleeves mountedwithin said frame, and a plurality of transducers mounted withinindividually associated ones of said tubular sleeves.
 8. The inventionin claim 7 further characterized in that said transducers areelectromagnetic devices.
 9. The invention in claim 8 furthercharacterized in that said transducers are of the inertial type.
 10. Theinvention in claim 7 further characterized in that there are unfilledopenings between said tubular sleeves whereby the radiation resistanceon the transducers is effectively reduced.
 11. The invention in claim 10further characterized in that said transducers are sphericalelectromagnetic devices, each having a mounting structure comprising anequatorially positioned resilient belt, said belt being compressedbetween the wall of said sphere and said tubular sleeve.
 12. Theinvention in claim 11 still further characterized in that saidtransducers are of the inertial type.